ALBERT

Notes: Hunt (1999) derived a model for computation of stream flow depletion caused by pumping from a well near a stream that does not fully penetrate the aquifer. The model input includes the transmissivity (T) and the storativity (S) of the aquifer and the conductance of the stream bed (λ). We study the experimental and hydrogeological conditions for which drawdown analysis can be expected to produce T, S, and λ estimates that can be used in the model of Hunt (1999) to predict depletion with a specified accuracy. The study is conducted by using a combination of sensitivity analysis and uncertainty analysis. The analysis shows that it is always important to have an accurate estimate of λ in order to accurately predict depletion. Recommendations for the design of a pumping test near a stream are given to achieve this. The pumping well should be located close to the stream. Accurate drawdown measurements should be made both near the pumping well and near the stream, and the measurements should be used simultaneously to estimate T, S, and λ. The duration of the pumping test should be relatively long in order to obtain an accurate estimate of λ. A methodology is proposed in which a desired accuracy of either the estimates of T, S, and λ, or the stream flow predicted from these estimates, is used to estimate the duration of the pumping test. Some examples indicate that in many cases the duration of the test should be from an hour to one or two months. However, if S is large (∼10−1) and λ is small (10-7 m/s) then the duration should be from several months to years. This would be expensive and not practical, so other methods should be used to estimate λ in this situation.

Notes: Ground water inflows to drifts ranging from 700 to 1615 m below ground surface at the Con Mine, Yellowknife, Northwest Territories, Canada, were used to study deep hydrogeological flow regimes in Shield terrain. Salinity trends are due to mixing between low-TDS ground water and deep Ca(Na)-C1 brines (〉290 g/L) likely derived from Devonian sea water. C1-−δ18O relationships demonstrate that all inflows are a mixture of three distinct components: modern meteoric ground water (δ18O ∼−18.9 ± 0.1%o), brine (δ18O ∼−10%o), and an isotopically depleted water (δ18O ∼−28%o). The origin of this third endmember is attributed to glacial melt water injected into the subsurface during ablation of the Laurentide Ice Sheet at ca. 10 ka. A mechanism is proposed where high hydrostatic pressure in the ablation zone imposes strong downward gradients beneath the ice sheet margin. Numerical simulation with the SWIFT II finite-difference code recreates the observed salinity gradients within a modeled 50-year interval, corresponding with the rate of retreat of the ice sheet across the landscape at this time. The persistence of this melt water in the subsurface for some 10,000 years following retreat of the ice and decay of the steep hydraulic gradients highlights the importance of gradient, in addition to permeability, as a major control on ground water flow and transport in deep crystalline settings.

Notes: Partitioning tracer tests (PTTs) are being used in environmental systems for the detection and estimation of nonaqueous phase liquid (NAPL) saturations in contaminated aquifers. A series of such studies was recently conducted at Hill Air Force Base, Utah, in two hydraulically isolated test cells of an aquifer contaminated by light nonaqueous phase liquids (LNAPL). These experiments were performed before and after two remediation efforts, a complexing sugar flush (CSF) and a recirculating in-well aeration (IWA) system. The breakthrough curves obtained from monitoring tracer concentrations in the extraction wells indicated the presence of an immiscible phase, and the LNAPL saturation values determined from the pre- and post-PTTs allowed the estimation of remediation efficiencies for both test cells. These remediation efficiencies, a removal of 43% of the LNAPL for the CSF and an increase of 32% for the IWA system, are consistent with data obtained from cores collected from within the experiment zones. The apparent increase in contamination for the IWA cell is likely due to a significant change in the LNAPL distribution caused by the flow system associated with the IWA technology. Several factors influenced the interpretation of the PTT data. Physical heterogeneities at the site caused significant tailing of the tracer concentrations and required the use of a simple extrapolation technique to account for the concentrations below analytical quantification limits. Degradation affected selected nonreactive and reactive tracers, causing the overestimation and underestimation of LNAPL saturations, respectively.

Notes: A numerical model is used to predict flow along intervals between producing zones in open boreholes for comparison with measurements of borehole flow. The model gives flow under quasi-steady conditions as a function of the transmissivity and hydraulic head in an arbitrary number of zones communicating with each other along open boreholes. The theory shows that the amount of inflow to or outflow from the borehole under any one flow condition may not indicate relative zone transmissivity. A unique inversion for both hydraulic-head and transmissivity values is possible if flow is measured under two different conditions such as ambient and quasi-steady pumping, and if the difference in open-borehole water level between the two flow conditions is measured. The technique is shown to give useful estimates of water levels and transmissivities of two or more water-producing zones intersecting a single interval of open borehole under typical field conditions. Although the modeling technique involves some approximation, the principle limit on the accuracy of the method under field conditions is the measurement error in the flow log data. Flow measurements and pumping conditions are usually adjusted so that transmissivity estimates are most accurate for the most transmissive zones, and relative measurement error is proportionately larger for less transmissive zones. The most effective general application of the borehole-flow model results when the data are fit to models that systematically include more production zones of progressively smaller transmissivity values until model results show that all accuracy in the data set is exhausted.

Notes: More than 270 multirate single-hole pneumatic injection tests were conducted by Guzman et al. (1996) within six shallow vertical and inclined boreholes in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) near Superor, Arizona. The authors used steady-state formulae to obtain air permeability values for the rock based solely on late pressure data from each test. We developed pressure and pressure-derivative type-curves for the interpretation of transient data from these tests, which account for storage in the test interval due to air compressibility and skin effect due to damage to the surrounding rock. We applied our type-curves to pressure data from more than 40 of the tests to obtain information about air permeability, skin factor, phenomenology, and dimensionality of the flow regime on a nominal scale of 1 m in the immediate vicinity of each test interval. Our air permeabilities agreed well with those determined previously by means of steady- state formulae but correlated poorly with fracture density data. Nonlinear effects due to air compressibility were pronounced, but skin effect was generally small or nonexistent. Nonlinear two-phase flow and inertia were evident but did not have a significant impact on type-curve interpretation of the test data. Most of these data fit a continuum model of spherical airflow around the test interval, but a few fit a continuum model of radial flow or suggest the dominance of a single fracture. We take this to imply that fractures at the site are pneumatically well connected and can, for the most part, be treated as a porous continuum when analyzing airflow on scales of 1 m or more. This is supported by the finding that 1 m scale air permeability data from single-hole tests at the ALRS are amenable to geostatistical analysis (Chen et al. 2000), which views them as a sample from a random field defined over a continuum. That the same holds true on larger scale is indicated by our ability to interpret cross-hole pneumatic injection tests at the site by means of type-curves and inverse methods (Illman et al. 1998), which treat the entire rock mass as a porous continuum.

Notes: The suitability of common-offset ground-penetrating radar (GPR) to detect free-phase hydrocarbons in bedrock fractures was evaluated using numerical modeling and physical experiments. The results of one- and two-dimensional numerical modeling at 100 megahertz indicate that GPR reflection amplitudes are relatively insensitive to fracture apertures ranging from 1 to 4 mm. The numerical modeling and physical experiments indicate that differences in the fluids that fill fractures significantly affect the amplitude and the polarity of electromagnetic waves reflected by subhorizontal fractures. Air-filled and hydrocarbon-filled fractures generate low-amplitude reflections that are in-phase with the transmitted pulse. Water-filled fractures create reflections with greater amplitude and opposite polarity than those reflections created by air-filled or hydrocarbon-filled fractures.The results from the numerical modeling and physical experiments demonstrate it is possible to distinguish water-filled fracture reflections from air- or hydrocarbon-filled fracture reflections, nevertheless subsurface heterogeneity, antenna coupling changes, and other sources of noise will likely make it difficult to observe these changes in GPR field data. This indicates that the routine application of common-offset GPR reflection methods for detection of hydrocarbon-filled fractures will be problematic. Ideal cases will require appropriately processed, high-quality GPR data, ground-truth information, and detailed knowledge of subsurface physical properties.Conversely, the sensitivity of GPR methods to changes in subsurface physical properties as demonstrated by the numerical and experimental results suggests the potential of using GPR methods as a monitoring tool. GPR methods may be suited for monitoring pumping and tracer tests, changes in site hydrologic conditions, and remediation activities.

Notes: Subsurface properties such as moisture content, hydraulic head, or chemical composition may vary markedly over short vertical distances in soil and ground water systems, but conventional samplers and sensors placed in vertical boreholes are often unable to resolve these variations. To improve the resolution of subsurface monitoring, we have developed a method for accessing the side-wall of a vertical or angled borehole at many discrete intervals along the entire length of the borehole. The method uses an access device that embeds sensors or sediment samplers laterally through the borehole sidewall into the undisturbed formation to distances slightly less than the diameter of the borehole. The access device can also obtain a core sample up to 15 cm long and 4 cm in diameter, and then insert a permeable sleeve for extracting fluid samples (water, gas, nonaqueous phase liquids). The system has been used under field conditions in the United States and Denmark to place electrodes capable of measuring water content (using time domain reflectometry [TDR] waveguides), Eh (using platinum electrodes), or electrical resistivity (using a four-conductor electrode). At one site, as many as 22 water samplers and 19 resistivity electrodes were installed in a single borehole at vertical spacings as close as 7 cm. This approach was used to install horizontally oriented TDR waveguides at depths greater than 10 m, thereby extending the TDR technique to the study of deep vadose zones. Other applications include measurement of in situ Eh at a site where strong chemical oxidants were injected to remediate sediments contaminated by organic chemicals.

Notes: Nitrate is now recognized as a widespread ground water contaminant, which has led to increased efforts to control and mitigate its impacts. This study reports on the long-term performance of four pilot-scale field trials in which reactive porous barriers were used to provide passive in situ treatment of nitrate in ground water. At two of the sites (Killarney and Borden), the reactive barriers were installed as horizontal layers underneath septic system infiltration beds; at a third site (Long Point), a barrier was installed as a vertical wall intercepting a horizontally migrating septic system plume; and at the fourth site (North Campus), a barrier was installed as a containerized subsurface reactor treating farm field drainage water. The reactive media consisted of 15% to 100% by volume of waste cellulose solids (wood mulch, sawdust, leaf compost), which provided a carbon source for heterotrophic denitrification. The field trials have been in semicontinuous operation for six to seven years at hydraulic loading rates ranging from six to 2000 L/day. Trials have been successful in attenuating influent NO3- (or NO3-+ NH4+ at Borden) concentrations averaging from 4.8 mg/L N at North Campus to 57 mg/L N at Killarney, by amounts averaging 80% at Killarney, 74% at Borden, 91 % at Long Point, and 58% at North Campus. Nitrate consumption rates were temperature dependent and ranged from 0.7 to 32 mg L N/day, but did not deteriorate over the monitoring period. Furthermore, mass-balance calculations indicate that carbon consumption by heterotrophic denitrification has so far used only about 2% to 3% of the initial carbon mass in each case. Results suggest that such barriers should be capable of providing NO3- treatment for at least a decade or longer without carbon replenishment.Reactive barriers have now been used to treat nitrate contamination from a variety of sources including septic systems, agricultural runoff, landfill leachate, and industrial operations. This demonstration of successful long-term operation should allow this technology to become more widely considered for nitrate remediation, particularly at sites where passive treatment requiring a minimum of maintenance is desired.

Notes: Integration of lithologic logs, geophysical logs, and hydraulic tests is critical in characterizing heterogeneous aquifers. Typically only a limited number of aquifer tests can be performed, and these need to be designed to provide hydraulic properties for the principle aquifers in the system. This study describes the integration of logs and aquifer tests in the development of a hydrostratigraphic model for the surficial aquifer system in and around Big Cypress National Preserve in eastern Collier County, Florida. Borehole flowmeter tests provide qualitative permeability profiles in most of 26 boreholes drilled in the study area. Flow logs indicate the depth of transmissive units, which are correlated across the study area. Comparison to published studies in adjacent areas indicates that the main limestone aquifer of the Tamiami Formation in the study area corresponds with the gray limestone aquifer in western Dade County and the water table and lower Tamiami Aquifer in western Collier County. Four strategically located, multiwell aquifer tests are used to quantify the qualitative permeability profiles provided by the flowmeter log analysis. The hydrostratigraphic model based on these results defines the main aquifer in the central part of the study area as unconfined to semiconfined with a transmissivity as high as 30,000 m2/day. The aquifer decreases in transmissivity to less than 10,000 m2/day in some parts of western Collier County, and becomes confined to the east and northeast of the study area, where transmissivity decreases to below 5000 m2/day.

Notes: During the first cycle of the National Water Quality Assessment (1992–1996), ground water in 20 of the nation's major hydro-logic basins was analyzed for 90 pesticide compounds (pesticides and degradates). One or more of the pesticide compounds examined were detected at 48.4% of the 2485 ground water sites sampled. However, approximately 70% of the sites where pesticides were detected, two or more pesticide compounds analyzed were present–documenting the prevalence of pesticide mixtures in ground water. The pesticide concentrations encountered were generally low, with the median total concentration (summation of concentrations for the 90 pesticide compounds) being 0.046 μg/L. Pesticides were commonly detected in shallow ground water beneath both agricultural (60.4%) and urban (48.5%) areas. This is an important finding because, although agricultural activities have long been associated with pesticide contamination, urban areas have only recently been recognized as a potential source of these types of compounds. Pesticides with higher frequencies of detection were generally those with more extensive use, greater environmental persistence, and greater mobility in ground water (lower soil-water partition coefficients).

Notes: Field tests were conducted to investigate skin effects in multilevel ground water samplers installed with direct push methods. Tests consisted of determining hydraulic conductivity values using slug tests in five multilevel samplers before, during, and after development. Development was conducted using a minisurge block tool designed for this study. Post-development hydraulic conductivity values were found to be 3.2 to 9.6 times greater than predevelopment values. The largest differences were observed in multilevel samplers positioned in low permeable materials that were installed in preprobed holes advanced by hydraulic hammering and pushing as opposed to probes situated in higher permeable materials that were installed in cored holes. The presence and magnitude of the skin effects, as well as the reduction of these effects by rigorous development, were confirmed through comparisons with steady-state pumping tests conducted in multilevel samplers and previously conducted slug tests in conventional monitoring wells. These results indicate the need for rigorous development prior to hydraulic conductivity testing in ground water samplers installed with direct push methods.

Notes: Monte Carlo simulation is used to examine the utility of simple (homogeneous/isotropic) models for the delineation of wellhead protection areas (WHPAs) in heterogeneous, statistically anisotropic, unconfined aquifers. Capture zone geometry is investigated under two hydrologic conditions: regional gradient negligible compared to pumping and significant regional gradient compared to the local gradient due to pumping. Variability of capture zone geometry is quantified in terms of simple measurements (maximum transport distance and maximum width of the capture zone) and the probability of inclusion in the capture zone of a series of hypothetical contaminant source locations. These results indicate that substantial uncertainty exists in the delineation of a wellhead capture zone. This uncertainty is related, principally, to flow that occurs along preferential pathways associated with zones of high hydraulic conductivity. It is suggested that these uncertainties can be accommodated in one of three fashions depending on the value of land, the benefit of wellhead protection, and the availability of funds to perform more thorough field assessment. The first option is to modify the estimated capture zone by a factor of safety. In the absence of significant regional flow, a factor of safety of approximately three applied to the calculated fixed radius estimate of the capture zone appears to be sufficient for the conditions simulated in this project. In the case of a capture zone within a significant regional gradient, uncertainty in the location of the real capture zone will require safety factors on the width of the estimated capture zone on the order of 30, with a safety factor of approximately three in the longitudinal direction. The second option reduces these factors of safety by increasing the risk that the estimated capture zone will not cover the entire area of the real capture zone. Work available in the literature provides insight into the trade-off between the size of the estimated capture zone and the risk endured. The third option is to reduce uncertainty in the vicinity of a wellhead through additional expenditure to allow the application of classic hydrologic techniques involving conditioning or inverse modeling.

Notes: Numerical simulations were used to examine the effects of heterogeneity in hydraulic conductivity (K) and intrinsic biodegradation rate on the accuracy of contaminant plume-scale biodegradation rates obtained from field data. The simulations were based on a steady-state BTEX contaminant plume undergoing biodegradation under sulfate-reducing conditions, with the electron acceptor in excess. Biomass was either uniform or correlated with K to model spatially variable intrinsic biodegradation rates. A hydraulic conductivity data set from an alluvial aquifer was used to generate three sets of 10 realizations with different degrees of heterogeneity, and contaminant transport with biodegradation was simulated with BIOMOC. Biodegradation rates were calculated from the steady-state contaminant plumes using decrease in concentration with distance downgradient and a single flow velocity estimate, as is commonly done in site characterization to support the interpretation of natural attenuation. The observed rates were found to underestimate the actual rate specified in the heterogeneous model in all cases. The discrepancy between the observed rate and the “true” rate depended on the ground water flow velocity estimate, and increased with increasing heterogeneity in the aquifer. For a lognormal K distribution with variance of 0.46, the estimate was no more than a factor of 1.4 slower than the true rate. For an aquifer with 20% silt/clay lenses, the rate estimate was as much as nine times slower than the true rate. Homogeneous-permeability, uniform-degradation rate simulations were used to generate predictions of remediation time with the rates estimated from the heterogeneous models. The homogeneous models generally overestimated the extent of remediation or underestimated remediation time, due to delayed degradation of contaminants in the low-K areas. Results suggest that aquifer characterization for natural attenuation at contaminated sites should include assessment of the presence and extent of, and contaminant concentrations in, low-permeability areas of an aquifer.

Notes: Knowing vertical conductivities of thick aquitards is important for simulating regional or local ground water flow. These hydraulic parameters are best determined by calibration of these models. Collection of reliable data from heavily pumped ground water reservoirs can be problematic. Another way to infer these hydraulic parameters is the simultaneous interpretation of different parts of a multiple pumping test using an inverse numerical model. Here, the drawdowns observed in the indirectly pumped aquifers allow us to deduce the vertical conductivities of the aquitards with an equivalent accuracy as the horizontal conductivities of the aquifers. When the hydraulic resistances of the aquitards are too large, it becomes impossible to induce observable drawdowns in the indirectly pumped aquifers, and the inverse model loses valuable input data. Observing vertical gradients in and over the aquitards in case of important vertical flow can make up for this loss. These data allow us to estimate the specific discharges through the successive parts of the aquitard and hence their hydraulic resistances. This useful information allows us to consider all these hydraulic resistances in one group of hydraulic parameters during the simultaneous interpretation of the multiple pumping test using an inverse numerical model. As this procedure reduces the number of identified parameter groups, it enhances their reliability. In this paper it is demonstrated how this reliability can be deduced from a number of statistical parameters that can also be used in the collinear diagnosis. The proposed interpretation procedure is demonstrated on a triple pumping test performed in and around the most important aquitard in Flanders, Belgium. The obtained results should first be integrated in a local ground water flow model. By the performance of a number of such integrated hydrogeological studies, the lateral change in the hydraulic properties of this aquitard can be evaluated.

Notes: This study investigated the unsaturated zone properties that affect ground water reaeration (i.e., the diffusive flux of oxygen through the unsaturated zone and into an aquifer system). Laboratory column experiments were undertaken to quantify oxygen flux into anaerobic ground water as a function of soil type, soil water content, soil oxygen demand, and unsaturated zone thickness. Soils used in these studies included coarse sand, sand, loamy fine sand, fine sandy loam, silt, silty clay loam, bentonite, kaolin, and peat. The results showed that reaeration flux rates ranging from 11,000 to 12,000 mg/m2-day were possible under conditions typical of the natural environment. Soil water content provided the greatest resistance to oxygen transport in the unsaturated zone while the remaining factors of soil type, soil oxygen demand, and unsaturated zone thickness did not significantly inhibit reaeration flux into ground water. An unsaturated zone transport model based on Fick's second law and gas-liquid interfacial mass transport was developed and showed good agreement with the experimental results derived from the column studies. The results of this study suggest that unsaturated zone reaeration of anaerobic ground water may be a contributing factor in controlling the steady–state size and shape of hydrocarbon plumes and incorporating reaeration into ground water models—based on unsaturated zone properties—may allow for a more accurate representation of bioattenuation reactions taking place in subsurface environments.

Notes: Book reviewed in this article:“Natural Attenuation of Fuels and Chlorinated Solvents in the Subsurface” by Todd H. Wiedemeier, Hanadi S. Rifai, Charles J. Newell, and John T. Wilson Reviewed by Eric Nichols.

Notes: Water discharging from numerous tunnels constructed during mining in the Wasatch Mountains near Salt Lake City, Utah, flows into nearby creeks. Disputes over ownership of water feeding the creeks have resulted in extensive litigation. In the course of a legal dispute over ownership of outflow from the Kentucky-Utah (K-U) Tunnel we evaluated the patterns and rates of ground water flow using an integrated study of the geology, chemistry, isotopes, and chlorofluorohydrocarbon (CFC) composition of the water. A sequence of sedimentary rocks with a range of hydraulic conductivity values has been folded, faulted, intruded by igneous rocks, and then eroded to create the rough topography of the Wasatch Mountains. The similarity of composition among tunnel discharge, springs, and base flow in the creek indicates that the creek is fed by ground water circulating in local, shallow flow systems. Results of numerical simulations of ground water flow indicate that the K-U Tunnel likely intercepts ground water that, in the absence of the tunnel, would ultimately flow in the subsurface to Big Cottonwood Creek. CFC and tritium contents of the water indicate flow weighted average ground water travel times range from four to 23 years and support our conclusion that water discharging from the tunnel is moving within a shallow ground water flow system. Despite sparse data, the scientific understanding was deemed sufficient for the judge to rule that owners of the surface water also own the tunnel discharge because, in the absence of the tunnel, this water would supply the stream.

Notes: Based on the column tracer test, we developed a research method to determine the hydraulic conductivity and effective porosity of saturated clays under low hydraulic gradients or small flow rates. Derived from Darcy's law and the solute transport equation, this method evaluates the hydraulic conductivity through measuring solute concentrations rather than by measuring flow rates. And the effective porosity is determined by applying an analytical solution of the one-dimensional uniform flow equation.Two types of experimental data drawn from a review of the literature and four sorts of accuracy test data carried out in the laboratory are used to examine the proposed method. The reproducibilities of accuracy tests for hydraulic conductivity determination indicate a consistency within a 5.5% error margin. The experimental results further indicate that hydraulic conductivities determined using the tracer method are more precise than those from the conventional flowmeter method. In addition, with the assistance of the proposed tracer method, we argue that the effective porosities may be overestimated as shown in the drawn example cases due to mistaking total pore volumes as the effective pores. The results of the accuracy tests further indicate that the effective porosities of saturated clay specimens are significantly smaller than the total porosities, at least when the samples are allowed to swell freely.

Notes: Over the last two decades, there has been a growing recognition of the importance of evaluating ground water's contribution to lakes. As a result, a number of techniques have been developed for measuring hydraulic properties across lake bottoms, primarily in the littoral zone. However, for larger, deeper lakes such as the Great Lakes these techniques are impractical in the profundal zone. As a result, many water balance calculations in these settings omit the ground water component altogether owing to the difficulties encountered in making the necessary hydraulic observations across the deeper lake bottom sediments.In this study, a methodology is developed for determining the flux into large, deep lakes using a combination of existing and recently developed techniques. The methodology is applied to the Hamilton Harbor, a natural bay at the western end of Lake Ontario, to estimate the ground water contribution to the harbor's water budget. Hydraulic gradients were monitored in 37 piezometers within the harbor during 1993 and 1994. Calculated hydraulic gradients, along with sediment hydraulic conductivities measured or estimated using a number of techniques, are used to estimate ground water flux to the harbor through Thiessen polygon weighting.Measured hydraulic gradients ranged from −0.333 to 0.430, the majority being upward indicating ground water discharge conditions. Gradients were varied across the harbor and increased in magnitude closer to shore. The total ground water contribution to the harbor was estimated to be 2.1 × 107 m3/yr. Compared with other hydrological components, ground water was slightly larger than the yearly precipitation input, and approximately 8 % of the total surface inflows to the harbor, and 2 % of the total surface outflow through the Burlington ship canal, which connects the harbor to Lake Ontario.The computed ground water flux suggests that despite the fact that ground water flux to large lakes may typically be a smaller input than surface water inputs, it is still a significant component of the overall water budget and should not be automatically omitted from water balance calculations.

Notes: The fate of disinfection byproducts during aquifer storage and recovery (ASR) is evaluated for aquifers in southern Nevada. Rapid declines of haloacetic acid (HAA) concentrations during ASR, with associated little change in Cl concentration, indicate that (HAAs) decline primarily by in situ microbial oxidation. Dilution is only a minor contributor to HAA concentration declines during ASR. Trihalomethane (THM) concentrations generally increased during storage of artificial recharge (AR) water and then declined during recovery. The decline of THM concentrations during recovery was primarily from dilution of current season AR water with residual AR water remaining in the aquifer from previous ASR seasons and native ground water. In more recent ASR seasons, for wells with the longest history of ASR, brominated THMs declined during storage and recovery by processes in addition to dilution. These conclusions about THMs are indicated by THM/Cl values and supported by a comparison of measured and model predicted THM concentrations. Geochemical mixing models were constructed using major-ion chemistry of the three end-member waters to calculate predicted THM concentrations. The decline in brominated THM concentrations in addition to that from dilution may result from biotransformation processes.

Notes: Principal component analysis was used to define patterns in water table hydrographs at four small, lake-watershed research sites in the United States. The analysis provided insights into (1) characteristics of ground water recharge in different parts of the watersheds; (2) the effect of seepage from lakes on water table fluctuations; and (3) the effect of differences in geologic properties on water table fluctuations. At two sites where all of the water table wells were completed in permeable deposits, glacial out-wash in Minnesota and dune sand in Nebraska, the patterns of water table fluctuation primarily reflected timing and magnitude of recharge. The water table had more frequent and wider ranges in fluctuations where it was shallow compared with where it was deep. At two sites where the water table wells were completed in sand or till, a glaciated mountain valley in New Hampshire and stagnation moraine in North Dakota, the patterns of water table fluctuations were strongly related to the type of geologic unit in which the wells are completed. Furthermore, at the New Hampshire site, the patterns of water table fluctuations were clearly different for wells completed in sand downgradient of a lake compared with those completed in sandy terraces on a mountain-side. The study indicates that principal component analysis would be particularly useful for summarizing large data sets for the purpose of selecting index wells for long-term monitoring, which would greatly reduce the cost of monitoring programs.

Notes: Deterministic and stochastic methods of three-dimensional hydrogeologic modeling are applied to characterization of contaminated Eocene aquifers at the Savannah River Site, South Carolina. The results address several important issues, including the use of multiple types of data in creating high-resolution aquifer models and the application of sequence-stratigraphic constraints. Specific procedures used include defining grid architecture stratigraphically, upscaling, modeling lithologic properties, and creating multiple equiprobable realizations of aquifer stratigraphy. An important question answered by the study is how to incorporate gamma-ray borehole-geophysical data in areas of anomalous log response, which occurs commonly in aquifers and confining units of the Atlantic Coastal Plain and other areas. To overcome this problem, gamma-ray models were conditioned to grain-size and lithofacies realizations. The investigation contributes to identifying potential pathways for downward migration of contaminants, which have been detected in confined aquifers at the modeling site.The approach followed in this investigation produces quantitative, stratigraphically constrained, geocellular models that incorporate multiple types of data from borehole-geophysical logs and continuous cores. The use of core-based stochastic realizations in conditioning deterministic models provides the advantage of incorporating lithologic information based on direct observations of cores rather than using only indirect measurements from geophysical logs. The high resolution of the models is demonstrated by the representation of thin, discontinuous clay beds that act as local barriers to flow. The models are effective in depicting the contrasts in geometry and heterogeneity between sheet-like nearshore-transgressive sands and laterally discontinuous sands of complex shoreline environments.

Notes: To model ground water flow in a shallow aquifer underlying a salt marsh, the impact of total stress changes due to tidal loading of the marsh surface on the piezometric head within the aquifer must be considered. If the deformation of the porous medium is negligible, a ground water flow model may be modified to account for the changing total stress. The incorporation of these modifications into the U.S. Geological Survey (USGS) numerical model SUTRA is discussed, and example problems for which analytic solutions are available are presented to demonstrate the behavior and validity of the modified model. In addition, the form of the specific pressure storativity term used in the SUTRA model is discussed and modifications of this form are suggested.

Notes: The relationship between the ground water sample and the surrounding ground water environment is controlled by wellborn flow and mixing that carry ground water to the sampling device. Controlled laboratory conditions and uniform inflow and inflowing concentration allowed detailed investigations of wellbore concentration responses to pump-induced flow and mixing independent of external complicating influences. During pumping from the top of the screen, the results agreed with wellbore flow theory except for high-frequency, high-amplitude concentration fluctuations and mixing with the blank casing due to slight density contrasts (∼0.005%). Concentration fluctuations indicated partial mixing of ground water with well water and declining concentrations in the blank casing indicated that convection due to the slight density contrast overcame pump-induced velocities. During pumping from the bottom of the well, opposing forces of buoyancy and pumping produced small-scale cumulative mixing as the ground water from different portions of the screen approached the pump. Even during low-flow pumping (i.e., 220 mL/min), 5% of the water was prepurge well water after five well volumes were pumped. In the field the density contrasts that resulted in these mixing processes might be caused by a slight cooling of the wellhead or a slight increase in temperature with depth (〉0.2°C/m). If the temperature gradient was more pronounced, the convective mixing of screen water with the potentially altered casing water would be more thorough and sampling without mixing would not be possible. If the temperature gradient is stable or if the water in the blank casing is not significantly altered, casing water mixing will not present a problem.

Notes: Determining the discharge of ground water to Shingobee Lake (66 ha), north-central Minnesota, is complicated by the presence of numerous springs situated adjacent to the lake and in the shallow portion of the lakebed. Springs first had to be located before these areas of more rapid discharge could be quantified. Two methods that rely on the distribution of aquatic plants are useful for locating springs. One method identifies areas of the near-shore lakebed where floating-leaf and emergent aquatic vegetation are absent. The second method uses the distribution of marsh marigold (Caltha palustris L.) to locate springs that discharge on land near the shoreline of the lake. Marsh marigold produces large (2 to 4 cm diameter) yellow flowers that provide a ready marker for locating ground water springs. Twice as many springs (38) were identified using this method as were identified using the lack of near-shore vegetation. A portable weir was used to measure discharge from onshore springs, and seepage meters were used to measure discharge from near-shore springs. Of the total 56.7 L s−1 that enters the lake from ground water, approximately 30% comes from onshore and near-shore springs.

Notes: Steady-state analytical solutions are presented for estimating the ground water inflow rate to a mine pit that may contain a pit lake of finite depth. The solutions consider (1) the effect of decreased saturated thickness near the pit walls; (2) distributed recharge to the water table; and (3) upward flow through the pit bottom. While the solutions are not appropriate for all hydrogeologic situations, they are relevant to conditions encountered at many mine sites. An example calculation is presented for an actual mine pit containing a pit lake. The analytical solutions provide an estimated ground water inflow rate that is similar to the rate determined independently from a detailed water balance study.

Notes: Sea water intrusion has occurred on the east and southeast coasts of Laizhou Bay, China, since the 1970s (Wu et al. 1993). In 1981, on the adjacent south coast, the intrusion of salt water originating from brine was observed. In this area, the salt water intrusion was caused by the excessive pumping of fresh water in aquifers. Moreover, the simultaneous pumping of fresh water and salt water/brine formed a complicated ground water flow field. The data obtained from observation wells were used to analyze the origin of the salt water and brine, and the genesis of the bicarbonate/sodium water zone. All data suggest that the brine originated from ancient sea water, and that the bicarbonate/sodium water was formed by cation exchange. The variations of chemical compositions along representative flowpaths and the relationships between such variations and salt water intrusion were also discussed.

Notes: A method of computing the vertical distributions of solute concentrations in aquifer strata penetrated by an existing well or borehole can be developed from a simple solute mass/water balance approach. This approach allows refinement of site characterization without the cost of drilling additional wells or using other site characterization methods. This method also allows pre-diction of the water quality impacts of different well construction and screening options when the vertical distribution of flows from an aquifer and the aquifer water concentrations are known or can be estimated. Data requirements and methods of data collection for use of the technique are reviewed. Flowmeter or spinner logging surveys indicate the flow contribution of each portion of the well. When depth-specific samples are collected under the same pumping conditions as the spinner logging, a solute mass/water balance approach allows vertical concentration distributions in the aquifer to be estimated. Application to a well containing arsenic concentrations that approach the maximum contaminant level (MCL) of 50 μ/L indicates that water with arsenic concentrations of nearly 100 μ/L enters the well from a particular stratum.

Notes: Standard measurements of solute sorption to sediments are typically made on the 〈2 mm sediment fraction. This fraction is used by researchers to standardize the method and to ease experimental protocol so that large labware is not required to accommodate the gravel fraction (〉2 mm particles). Since sorption is a phenomenon directly related to surface area, sorption measurements based on the 〈2 mm fraction would be expected to overestimate actual whole-sediment values for sediments containing gravel. This inaccuracy is a problem for ground water contaminant transport modelers who use laboratory-derived sorption values, typically expressed as a distribution coefficients (Kd), to calculate the retardation factor (Rf), a parameter that accounts for solute-sediment chemical interactions. The objectives of this laboratory study were to quantify the effect of gravel on strontium Kd and Rf values and to develop an empirical method to calculate gravel-corrected Kdgc values for the study site (Hanford Site in Richland, Washington). Three gravel corrections, Kd values, were evaluated: a correction based on the assumption that the gravel simply diluted the Kd〈2mm and had no sorption capacity (Kdgc,g=0), a correction based on the assumption that the Kd of the intact sediment (Kdtot was a composite of the Kd〈2mm and the Kd〉2mm (Kdgc,g = x), and a correction based on surface area (Kdgc,surf). On average, Kd〈2mm tended to overestimate Kdtot by 28% to 47%; Kdgc,g = x overestimated Kdtot by only 3% to 5%; and Kdgc,g = 0 and Kdgc,surf underestimated Kdtot by 10% to 39%. Kdgc,g = x provided the best estimate of actual values (Kdtot); however, Kdgc,g = 0 was appreciably easier to acquire. Although other contaminants will likely have different gravel-correction values, these results have important implications regarding the traditional approach to modeling contaminant transport which uses Kd〈2mm values. Such calculations may overestimate the tendency of gravel-containing sediments to retard contaminant migration.

Notes: Analyses of ground water from wells and springs in the United States indicate a broad regional trend in preanthropogenic (36Cl/total Cl) ×1015 ratios in potable water. Coastal areas influenced by marine chloride have ratios less than 100. These ratios increase inland and reach a maximum of about 1400 in the central and northern Rocky Mountains. However, the magnitude of these regional variations is greatly exceeded at a local level if water samples are considered regardless of age or origin. Most local departures from regional trends can be attributed to mixing of ground water recharge with either 36Cl from nuclear fusion tests to produce higher ratios or mixing with sources of old chloride such as from evaporite minerals to produce lower ratios. A useful interpretation of 36Cl data is difficult unless the origin of the chloride is understood. The interpretation is an unusually difficult problem for water having low chloride concentrations. The assumption that low concentrations originate primarily from wet and dry atmospheric deposition can be questioned even for some dilute waters with less than 2.0 mg/L chloride. Chloride/bromide ratios in ground water can help decipher the origin of the chloride. The presence of nonatmospheric chloride can be identified in part by its association with chloride/bromide mass ratios greater than about 200 in coastal areas and 100 in inland areas. Accurate chloride and bromide analyses are recommended for virtually all studies of 36Cl in natural waters, and particularly those with low concentrations of chloride.

Notes: We present a novel, explicit closed form solution using the Lambert W function to a steady-state one-dimensional contaminant transport equation incorporating the Monod expression for biodegradation. This new solution overcomes some of the limitations associated with using the existing implicit solution and provides a simple method of describing biodegradation at contaminated sites using the Monod equation. The field-scale applicability of this approach was tested using data on phenol and toluene biodegradation from previously published studies. A review of published Monod kinetic parameters for hydrocarbon biodegradation indicated that in most cases, the half saturation coefficient was lower than 5 mg/L, suggesting that the commonly used first-order approximation is valid only at low substrate concentrations. While the Monod equation does not have this limitation and can usually provide an accurate description of biodegradation at contaminated sites, its nonlinear nature presents computational and parameter estimation difficulties compared with the linear and relatively simple first- and zero-order approximations. The explicit closed form solution to the Monod equation presented in this study greatly simplifies its use and appears to be an attractive approach to describe contaminant biodegradation in the field.

Notes: Two detailed vertical profiles through a complex plume of phenolic contaminants in a Triassic sandstone aquifer show that natural attenuation by biodegradation and dispersion is active but very slow. The plume has a microbially active aerobic and NO3 reducing fringe that is less than 2 m thick at both 150 and 350 m downstream of the source. The anaerobic core has evidence of active bacterial populations and degradation at total organic carbon (TOC) concentrations up to at least 1400 mg/L (1800 mg/L total phenolics), although gross half-lives are more than 50 years. There is evidence from the same locations of Mn, Fe, and SO4 reduction, with the latter inhibited by the pollutant matrix and not significant at concentrations more than 1000 mg/L TOC. Degradation of these contaminants in this aquifer is influenced by a range of environmental factors, including the chemical toxicity and pH of the contaminant matrix, and inputs of electron acceptors into the plume by dispersion. The results show that the plume is likely to grow under the present conditions, despite the biodegradable nature of the organic pollutants and availability of suitable electron acceptors. Vertical profiles have proved a cost-effective method of understanding the evolution of the plume.

Notes: Ground water flow modeling in a karst aquifer presents many difficulties. In particular, the hydrodynamic properties and the flow behavior can vary over time. History matching of transient-state conditions is required to test the accuracy of the model under varying hydrodynamic conditions. The objective of this study was to illustrate how transient-state conditions can be used to history match a ground water flow model of a large aquifer, the La Rochefoucauld karst (Charente, France). The model used a porous medium equivalent and was based on a steady-state calibration of hydraulic conductivities. The history match consisted of studying the simulated heads and spring flow rates to test the capacity of the model to reproduce different aspects of the aquifer behavior. The simulated heads and flow rates were analyzed as new data using correlation and spectral analyses to compare the temporal structures of the measured and simulated time series. The analyses provided information on the storage capacity of the aquifer, the input-output delays, the degree of correlation between input and output, and the length of the impulse response of the aquifer. These data were used to study the impact of the hypotheses underlying the model (hydraulic conductivities, storage coefficient, representation of rivers, use of a porous medium equivalent). The results show that the model adequately simulates the overall behavior of the studied aquifer. The model can be used under variable hydrodynamic conditions to simulate ground water flow on a regional scale. This case study illustrates how a complete history match of a simplified representation of reality can lead to an adequate mathematical tool.

Notes: Ongoing decline of water levels in the confined basalt aquifers of the Pullman-Moscow Basin of Washington and Idaho has prompted study of the timing, amount and distribution of recharge to the system. Previous radiocarbon ages indicate residence times on the order of 103 years and greater and suggest a low rate of recharge to the lower basalt aquifer since the end of Pleistocene time. By contrast, more recent hydrodynamic flow modeling studies invoke a larger Holocene recharge rate through the unconfined loess unit to the upper and lower basalt aquifers, which implies relatively short residence times (102 years). Stable isotopes were used to independently assess contrasting recharge models by comparing 18O/16O and D/H ratios of late-Holocene shallow ground water and deep ground water. Linear regression of local precipitation ratios yields δD = 6.9 δ18O −18.5. There is no evidence of fractionation of ground water ratios by recharge processes or water-rock interactions. Deep basalt ground water δ18O values are depleted by 0.4 to 4.9 per mil relative to shallow, recently recharged ground waters and have δ18O values statistically distinct from waters sampled from other stratigraphic units. These findings suggest that the deep waters in the basin were not precipitated under current climate conditions and that aquifer recharge rates to the deep basalt aquifer are substantially lower than have been recently estimated. This in turn suggests that a sustainable ground water exploitation scheme must reduce reliance on the deep ground water resource.

Notes: An assessment of a conventional soil vacuum extraction (SVE) system to remove perchloroethylene (PCE), a dense nonaqueous phase liquid (DNAPL), was conducted in a dewatered 9 × 9 m block of the surficial sand aquifer located at the Canadian Forces Base (CFB), Borden, Ontario. Approximately two years prior to the installation and operation of the SVE system, 1250 kg of PCE were released under saturated conditions into this experimental cell, forming a heterogeneous DNAPL distribution. Using ground penetrating radar and neutron probe data in conjunction with detailed soil core data, it was determined that approximately 600 kg of PCE were available for removal by the SVE system after dewatering. Performance assessment of this SVE system was based on soil core data and monitoring of in situ gas concentration and pressure, soil temperature, and moisture content.After the first 25 days of operation, the SVE system had removed 230 kg of PCE, during which time the off-gas concentration decreased by 70% from an initial concentration of close to 3500 ppmY After 432 days of operation, 63% (376 kg) of the estimated available mass was removed from the SVE target zone. In situ pressure measurements and sulphur hexafluoride tracer tests indicated that advective air movement occurred throughout the SVE target zone. Changes to the extraction well configuration and the use of short-screened extraction drive points did not increase the mass removal performance.After 250 days of operation, eight soil cores were removed from the test cell, and bulk soil concentration and soil moisture content measurements were performed at 0.05 m increments along the length of each core. These data revealed zones where significant quantities of PCE remained, even though the cumulative mass removal profile for the SVE system was nonzero asymptotic. In general, these zones coincided with higher moisture content zones reflective of the heterogeneous nature of this porous medium.This investigation provides insight into some of the limitations of conventional SVE technology to clean up a sand aquifer contaminated with a DNAPL, given a reasonably good estimate of the before-extraction mass conditions. In particular, the findings clearly show that the heterogeneous nature of the porous medium gives rise to a nonuniform moisture content, a heterogeneous pure phase distribution, and the development of preferential gas phase flow pathways. As a consequence, this will in general limit this technology to remove a considerable fraction of the DNAPL mass present. The results also shed light on the use of soil gas and bulk soil concentration data to provide a representative picture of the in situ mass distribution. And finally, an analysis of the system off-gas data showed that continuous rather than pulsed operation of the extraction system increased mass removal.

Notes: The projected growth of Hernando County, located along the coast of west-central Florida, has motivated the county and the Southwest Florida Water Management District to assess ground water resources in the area. As part of the Hernando County Water Resources Assessment Project, a computationally efficient modeling approach was developed to simulate regional ground water flow and salt water intrusion in two adjoining ground water basins in which Hernando County is located. In this modeling approach, the regional ground water flow simulation was divided into two stages: fresh water and sharp-interface simulations. The first stage was performed using a calibrated fresh water model. Following the fresh water simulation, a calibrated sharp-interface model was used to evaluate the potential extent of migration of the fresh water/salt water interface as a result of implementing ground water management scenarios. Predictive modeling results indicate that projected future ground water withdrawals will cause less than 0.6 m of additional drawdown within Hernando County. Results also suggest that by the year 2050 the fresh water/salt water interface will migrate landward less than 1.6 km from its 1994 position.

Notes: A barrier well line's capture width along its alignment is shown to depend only on total system flow rate QT, and aquifer back-ground flow rate per unit width Qo (prepumping gradient times transmissivity) under idealized horizontal flow: Irrespective of number of wells, the capture width is QT/2Q0, as for a hypothetical single well with flow rate QT. This result applies even to barrier lines with irregular well spacings and flow rates–and, in fact, to the capture width through the center of any pumping configuration that is symmetrical respecting a line perpendicular to uniform background flow. A flow-perpendicular line of regularly spaced, equal wells (“regular barrier line”) has the same capture width along its alignment as an identical well line parallel to background flow has through its center. In the general symmetrical case, the center capture width remains QT/2Q0 whether transverse well spacings are large (allowing contaminated ground water to escape through capture zone gaps), critical (with incipient gaps), or safely small. For design reference, critical spacings derived for regular barrier lines of N wells with N through 100 and N→∞ vary from Q/πQ0 (N = 2, previously known) to Q/2Q0 (N→∞), where Q is each well's flow rate. Incipient gaps at critical spacing occur always between the two center wells, or the center well and two adjacent wells. Spacings substantially smaller than critical are recommended for contaminant capture. Designs are illustrated that preclude capture zone gaps.

Notes: The use of physical and hydraulic containment systems for the isolation of contaminated ground water and aquifer materials associated with hazardous waste sites has increased during the last decade. The existing methodologies for monitoring and evaluating leakage from hazardous waste containment systems rely primarily on limited hydraulic head data. The number of hydraulic head monitoring points available at most sites employing physical containment systems may be insufficient to identify significant leaks from the systems. A probabilistic approach for evaluating the performance of containment systems, based on estimations of apparent leakage rates, is used to introduce a methodology for determining the minimum number of monitoring points necessary to identify the hydraulic signature of leakage from a containment system. The probabilistic method is based on the principles of geometric probability. The method is demonstrated using three-dimensional ground water flow modeling results of leakage through a vertical barrier. The results indicate that the monitoring point spacing used at many hazardous waste sites likely is inadequate to detect the hydraulic signatures of all but the largest leaks.

Notes: Concentrations of naturally occurring arsenic in ground water vary regionally due to a combination of climate and geology. Although slightly less than half of 30,000 arsenic analyses of ground water in the United States were 〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0017467X:GWAT589:les" location="les.gif"/〉 1 μg/L, about 10% exceeded 10 μg/L. At a broad regional scale, arsenic concentrations exceeding 10 μg/L appear to be more frequently observed in the western United States than in the eastern half. Arsenic concentrations in ground water of the Appalachian Highlands and the Atlantic Plain generally are very low (〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0017467X:GWAT589:les" location="les.gif"/〉 1 μg/L). Concentrations are somewhat greater in the Interior Plains and the Rocky Mountain System. Investigations of ground water in New England, Michigan, Minnesota, South Dakota, Oklahoma, and Wisconsin within the last decade suggest that arsenic concentrations exceeding 10 μg/L are more widespread and common than previously recognized.Arsenic release from iron oxide appears to be the most common cause of widespread arsenic concentrations exceeding 10 μg/L in ground water. This can occur in response to different geochemical conditions, including release of arsenic to ground water through reaction of iron oxide with either natural or anthropogenic (i.e., petroleum products) organic carbon. Iron oxide also can release arsenic to alkaline ground water, such as that found in some felsic volcanic rocks and alkaline aquifers of the western United States. Sulfide minerals are both a source and sink for arsenic. Geothermal water and high evaporation rates also are associated with arsenic concentrations 〈inlineGraphic alt="leqslant R: less-than-or-eq, slant" extraInfo="nonStandardEntity" href="urn:x-wiley:0017467X:GWAT589:les" location="les.gif"/〉 10g/L in ground and surface water, particularly in the west.

Notes: Two experiments were conducted on Long Key, Florida, United States, to examine the fate of waste water following sewage disposal in 10 to 30 m deep injection wells. This waste disposal practice introduces extraordinary amounts of nutrients into the ground water of the Florida Keys. In these experiments, artificial ground water tracers, sulfur hexafluoride (SF6) and radioiodine (131I) were used to determine transport rates and directions of soluble nonreactive substances injected into the saline ground water underlying the Keys. Two types of transport were observed: (1) rapid flow (0.20 to 2.20 m/hr) presumably due to the many conduits present in the limestone; and (2) slower flow (less than 0.003 to 0.14 m/hr) associated with the limestone's primary porosity. Vertical flow was comparable to horizontal flow due to either the density-driven buoyancy of the waste water plume or to preferential flowpaths that allow upward advection or combination of both. These experiments showed that conservative artificial tracers injected into the subsurface reach surface water in a matter of days and can remain in the immediate vicinity of the injection well for several months.

Notes: Air sparging experiments were conducted in a laboratory column to investigate air flow and mass transfer behavior in different types of sand at different air injection rates. Methyl tertiary butyl ether (MTBE) was applied as a tracer, and by measuring the volatilization and the mean air content during the experiments, the air flow pattern and its influence on mass transfer were assessed. The experimental results showed large differences among the sand types. In fine sand, the mean air content was high and the volatilization of MTBE was rapid with total recovery after a few hours. In coarse sand, the mean air content was low and the volatilization of MTBE was limited. The results indicate two different air flow distributions. In fine-grained materials, a uniform air distribution can be expected compared to coarse-grained materials where isolated air channels will limit the mass transfer. Afterwards, the experiments were simulated using the numerical multiphase flow code T2VOC, and the results compared to those obtained in the laboratory. The experiments with fine sand were simulated well, while for coarser sand types the volatilization was highly overestimated. The differences between model and laboratory results were mainly attributed to the nonuniformity of the air saturation and the neglection of kinetics in the mass transfer formulation.

Notes: Afield site was established in an area of glacial outwash near Des Moines, Iowa. Hydraulic conductivity (K) of the outwash was measured in various ways including six pumping tests and two natural-gradient Cl- tracer tests. The velocity of the conservative tracer was converted to K using measured gradients and effective porosity determined from two radial-convergent Cl- tracer tests.K values measured from the conservative tracer tests are approximately one-tenth to one-twentieth the average pumping-test value. Thus the K relevant to solute transport does not reflect the K measured by pumping tests. This discrepancy may be caused by the different scale and dimensionality of the two test types. Dispersion may prevent solutes from flowing exclusively within smaller high-conductivity paths which strongly affect the K measured by pumping tests.

Notes: This study concerns the problem of pump position in landfill monitoring wells and the correct time to stop purging and start sampling. Literature purge volume determination methods fail to use sufficient analyte values for comprehensive study of the stabilization plateau. From field studies it is recommended that prior to commencement of a sampling program a purge study be undertaken to determine pump position and optimum purge volume by conducting (1) a preliminary vertical electrical conductivity (EC) profile: (2) vertical profiling of formation water EC values by low-flow pumping at 0.5 m vertical intervals-while also noting EC vertical profile values to assure minimal effect on the water column water quality; and (3) a low-flow, purge EC stabilization test with the pump positioned at highest formation water EC values. Sufficient EC values are needed to fulfill Conover's (1980) nonparametric tolerance interval sample size criteria, and to graphically delineate the stabilization plateau. This information will allow you to choose an optimum purge volume that allows a margin for error.

Notes: Recovery wells remain the principle technology for removal of free-product hydrocarbon liquids from the subsurface. This paper presents simple models for estimating hydrocarbon recovery rates using wells and vacuum-enhanced systems. Use of LNAPL volume balance between LNAPL recovery rate and formation free-product volume leads to development of algebraic equations that can be used to estimate recovery times. Selection of model parameters is discussed, model comparisons are made, and applications are presented for design and analysis of recovery systems using wells. Model validation is also discussed.

Notes: Natural attenuation landfill sites continue to be the preferred method of domestic waste disposal in the Precambrian Shield regions of Ontario due to economic factors. The main challenge in siting these landfills is ensuring that there will be no adverse impact on off-site water resources. Impact risk assessments are generally based on estimated volumes and strengths of chloride in the leachate. While volumes can be estimated using simple water balances, peak chloride concentration predictions are based on judgment and are quite variable. Since design chloride strengths dictate the size of the required attenuation zone, overestimating concentrations will typically make it impossible to find a suitable site, while underestimating concentrations increases the potential for adverse off-site impacts occurring.Hydrogeological data from active and closed landfills in the Precambrian Shield region were collected to help develop a reliable method of predicting peak chloride concentrations in leachate. This study focused on 21 sites located on relatively permeable sandy soils since landfills underlain by low permeability clayey soils retain leachate similar to lined facilities.Linear regression analyses were conducted to determine if source chloride concentrations at the “sand” sites are significantly influenced by waste thickness, fill area, waste volume, waste deposition rate, hydraulic conductivity, upgradient flow length, depth to the water table, and moisture surplus.A strong relationship (R = 0.957) was found to exist between source chloride concentrations and waste volume. This empirical volume versus chloride regression equation can be used as the basis for establishing design chloride concentrations at new natural attenuation landfills developed over sandy soils in the Precambrian Shield regions of Ontario. An alternative risk assessment approach is required for sites developed over clay soils.

Notes: The use of decision analysis (DA) has been proposed as a technique for selecting from among alternative designs for subsurface remediation. To assess the ability of DA to generate consistent decisions for the widely practiced pump-and-treat (PAT) strategy, 27 candidate PAT designs were compared for a case study site. The sensitivity of the alternative selection to various modeling assumptions was examined, including the complexity of the site-specific numerical models, the assumed degree of aquifer heterogeneity, the manner of defining failure, and the assumed cost of failure. The initial net-present-worth analysis resulted in the selection of one of two designs that included injection wells for effluent disposal and hydraulic control. However, when the injection wells were excluded from consideration, the selection from a diverse set of alternative PAT designs was highly sensitive to the particular modeling assumptions. In general, the practical usefulness of the DA approach is dependent on the ability to characterize the nature and probability of system failure.

Notes: An electrical conductivity probe, designed for use with “direct push” technology, has been successfully used to locate buried drums, contaminant plumes, and to precisely locate and characterize a previously installed permeable reactive iron wall. The conductivity probe was designed to characterize various soil and sediment types as it was driven through the dry and saturated soil matrix; however, its ability to locate and characterize subsurface anomalies may also prove to be a valuable asset. The probe uses an electrical field that works like and mimics the results from an oilfield engineering tool called the Wenner array. This electrical field array penetrates into the soil matrix to measure the electrical conductivity of the soil matrix surrounding the probe to a radius of about two to four inches. This tool has allowed operating personnel to verify the location of manmade or natural subsurface anomalies with precision.

Notes: The installation of gas-filled diffusion samplers into small-diameter boreholes results in a significant reduction of the dissolved gas concentration around the sampler. In aquifers where the diffusive flux of solutes outpaces advective transport, the process that governs the equilibration time of a sampler is the resupply of solutes by diffusion from the aquifer. We have derived a solution that can be used to estimate the time required for a diffusion sampler to reach equilibrium with the dissolved gas concentration in the aquifer, where diffusion is the only solute transport mechanism. Thus the solutions provide equilibration times for cells placed in aquifers where diffusion dominates and maximum equilibration times for cells placed in aquifers where advection can also resupply solutes. The solutions are generic and are functions of nondimensionalized variables, therefore providing estimates of equilibration times for any type of solute, sampler volume, bore dimensions, and aquifer porosity. Examples are given for various sized gas-filled helium samplers placed in boreholes of different radii.

Notes: Free-phase DNAPL recovery operations are becoming increasingly prevalent at creosote-contaminated aquifer sites. This paper illustrates the potential of both classical and innovative recovery methods. The UTCHEM multiphase flow and transport numerical simulator was used to predict the migration of creosote DNAPL during a hypothetical spill event, during a long-term redistribution after the spill, and for a variety of subsequent free-phase DNAPL recovery operations. The physical parameters used for the DNAPL and the aquifer in the model are estimates for a specific creosote DNAPL site. Other simulations were also conducted using physical parameters that are typical of a trichloroethene (TCE) DNAPL. Dramatic differences in DNAPL migration were observed between these simulations.

Notes: Low-flow ground water sampling methodology can minimize well disturbance and aggravated colloid transport into samples obtained from monitoring wells. However, in low hydraulic conductivity formations, low-flow sampling methodology can cause excessive drawdown that can result in screen desaturation and high ground water velocities in the vicinity of the well, causing unwanted colloid and soil transport into ground water samples taken from the well. Ground water velocities may increase several fold above that of the natural setting. To examine the drawdown behavior of a monitoring well, mathematical relationships can be developed that allow prediction of the steady-state drawdown for constant low-flow pumping rates based on well geometry and aquifer properties. The equations also estimate the time necessary to reach drawdown equilibrium. These same equations can be used to estimate the relative contribution of water entering a sampling device from either the well standpipe or the aquifer. Such equations can be useful in planning a low-flow sampling program and may suggest when to collect a water sample. In low hydraulic conductivity formations, the equations suggest that drawdown may not stabilize for well depths, violating the minimal drawdown requirement of the low-flow technique. In such cases, it may be more appropriate to collect a slug or passive sample from the well screen, under the assumption that the water in the well screen is in equilibrium with the surrounding aquifer.

Notes: Remediation of ground water pollution at old landfills with no engineered leachate collection system is a demanding and costly operation. It requires control of the landfill body, since the majority of the pollutants are still present in the landfilled waste for decades after the site has been closed. However removing the source is an attractive approach to managing leachate plumes. Natural attenuation has been implemented for petroleum hydrocarbon plumes and chorinated solvent plumes, primarily in the United States. Natural attenuation has not yet gained a foothold with respect to leachate plumes, however. Based on the experience gained from 10 years of research on two Danish landfills, it is suggested that natural attenuation is a feasible approach but is more complicated and demanding than in the case of petroleum hydrocarbons and chlorinated solvent.

Notes: A previous field demonstration project on nitrate-based bioremediation of a fuel-contaminated aquifer used short-screened clustered well points in addition to shallow (10 foot), conventional monitoring wells to monitor the progress of remediation during surface application of recharge. These well systems were placed in the center and at one edge of each of two treatment cells. One cell received recharge amended with nitrate (nitrate cell), and the other received unamended recharge (control cell). Data from the clustered well points were averaged to provide a mean estimate for comparison with the associated conventional monitoring well.Conservative tracer profiles were similar for each of the four systems, with better fits obtained for well systems located at the edge of the treatment cells. However, aromatic hydrocarbon and electron acceptor profiles varied greatly for the two center well systems, with the conventional monitoring well data suggesting that remediation was proceeding at a much more rapid rate than indicated by the cluster well points. Later tests with an electromagnetic borehole flowmeter demonstrated a significant vertical flow through the well-bore of the conventional monitoring well under simulated operating conditions. This created an artifact during sampling, thought to arise from preferential flow of recharge water from the water table to deeper portions of the contaminated zone resulting in several effects, including an actual decreased residence time of water sampled by the conventional well. These data provide additional evidence that conventional monitoring wells may be inadequate for monitoring remediation in the presence of significant vertical hydraulic gradients, even for fairly shallow homogeneous aquifers.

Notes: Development agencies who work in the water sector often implement their aims using projects which provide basic physical infrastructure. The provision of adequate and appropriate water systems and services relies upon the availability of valid design data, such as per capita consumption and peak-flow factors, yet many project designs are based upon broad assumptions or inappropriate data. This paper presents the methods and results from a project to collect domestic water consumption data from rural water systems in Guatemala. General observations on the implications of the results are discussed together with the role which simple data collection can have within development projects.

Notes: This paper analyses the environmental impacts of a large-scale sewage-treatment plant and identifies opportunities for environmental performance improvements over the three life-cycle phases of construction, operation and demolition. Results show that nearly 70% of energy consumption occurred during the operation phase, which was lower than the normal energy consumption level of the conventional activated-sludge process. The results of the life-cycle assessment may have implications in material selection, energy management, process design, supplier and contractor management, and environmental policy making.

Notes: Technologies for the thermochemical treatment of sewage sludge are appraised with reference to their efficacy in terms of (a) operational parameters, (b) pre-and post-treatment requirements, and (c) the extent of their use for the application. Particular attention is given to the characteristics of the solid and gaseous products and how such characteristics affect post-treatment, reuse or disposal – this being dictated by environmental legislation.Of specific interest is the off-gas treatment, which can contribute significantly to operating and capital costs of incineration. On the other hand, gasification and pyrolysis generate a combustible product for use in heat and/or power generation. Available information, although limited, suggests that gasification offers significant advantages over the conventional incineration process, yielding a combustible gaseous product (which can be directly utilised on site) and a solid char product (which is less prone to leaching of toxic metals).

Notes: During recent decades, the anaerobic digestion process has been extensively studied and various methods for process enhancement have been explored. These methods include heat treatment, alkali addition, phase separation and membrane enhancement. In general, whilst technically feasible, the methods have not proved to be economically competitive.The pretreatment of sludges with high-power ultrasound has been investigated as part of a twelve-month research project funded by six UK water utilities. Cell lysis and particle-size reduction (caused by ultrasonic cavitation) are thought to be the key parameters through which the digestion process is enhanced. A series of laboratory-scale anaerobic digesters has been operated and significant increases in biogas yield have been noted following ultrasonication. Experiments have been completed with a variety of ultrasonic devices (of different geometries and construction materials) and sludge types (e.g. primary, secondary and co-settled). Current experiments are investigating methods through which the technology can be successfully scaled-up and applied on a full-scale plant.

Notes: Book Reviews in This Article:〈section xml:id="abs1-1"〉〈title type="main"〉Chalk Rivers: Nature Conservation and Management, by C. P. Mainstone.〈section xml:id="abs1-2"〉〈title type="main"〉Quaking Houses – Art, Science and the Community, by Penny Kemp and John Griffiths.

Notes: The quality of recreational waters has become of increasing importance and, where water-based activities are permitted, such waters should be required to comply with minimum standards to protect public health. At present, there are no freshwater sites in the UK which are identified as official bathing areas, despite a significant proportion being identified in other EU Member States and the increasing use of such waters for bathing. This paper presents a case for the official identification of three freshwater bathing sites which, traditionally, have been used for bathing for more than ninety years.

Notes: Source control is regarded as a key principle in supporting concepts of sustainable management and integrated pollution control for diffuse non-point stormwater runoff, with infiltration procedures being a prime component of such source-control systems. The potential conflicts between the benefits of groundwater recharge and the risks to long-term groundwater quality uncertainties associated with infiltration disposal are considered. The long-term performance of a number of infiltration systems is reported in terms of their pollutant removal efficiencies, and the utility of an infiltration acceptability matrix approach is reviewed.